MacBook Pro has become very very slow

Click on finder , take cursor on top menu bar > click on Go > hold option key and click on library , delete the complete contents of folders Caches , Cookies , Saved application state , logs to the trash .

Restart the Mac and then empty the trash , the new contents will be once again created in these folders .

Logs contents are removed in very rare scenarios but that can be done , consult this with apple support senior level advisors https://support.apple.com/en-in/contact

If your Mac is compatible with newer version always upgrade it , you can utilise the latest features for that version see this article https://www.apple.com/in/macos/how-to-upgrade/#hardware-requirements